RF resonators and filters

ABSTRACT

A filter package comprising an array of piezoelectric films sandwiched between lower electrodes and an array of upper electrodes covered by an array of silicon membranes with cavities thereover: the lower electrode being coupled to an interposer with a first cavity between the lower electrodes and the interposer; the array of silicon membranes having a known thickness and attached over the upper electrodes with an array of upper cavities, each upper cavity between a silicon membrane of the array and a common silicon cover; each upper cavity aligned with a piezoelectric film, an upper electrode and silicon membrane, the upper cavities having side walls comprising SiO 2 ; the individual piezoelectric films, their upper electrodes and silicon membranes thereover being separated from adjacent piezoelectric films, upper electrodes and silicon membranes by a passivation material.

BACKGROUND

Mobile phone users require quality reception and transmission over awide area. The quality of the radio frequency (RF) signal depends on theRF filters in the mobile phone. Each RF filter passes desiredfrequencies and rejects unwanted frequencies enabling band selection andallowing a mobile phone to process only the intended signal.

It has been estimated that by 2020, a shift to Carrier aggregation, 5Gand 4×4 MIMO could result in mobile phones requiring upwards of 100filters and a global market of 200 billion filters a year.

Acoustic resonators are a basic building block of RF filters andsensors. These typically include a piezoelectric electromechanicaltransduction layer which converts mechanical energy into electricalenergy. These resonators have to be cheap but reliable. The two mostcommon types of acoustic resonators are Surface Acoustic Wave Resonators(SAW) and Bulk Acoustic Wave Resonators (BAW).

In Surface Acoustic Wave resonators the acoustic signal is carried by asurface wave. In Bulk Acoustic Wave Resonators (BAW) the signal iscarried through the bulk of the resonator film. The resonant frequencyof both types of filter is a characteristic of its dimensions and of themechanical properties of the materials used in their construction.

The quality of a resonator is given by its Q factor. This is the ratioof the energy stored to the power dissipated. A high Q factor indicatesthat the filter loses little energy during operation. This translates toa lower insertion loss and a steeper skirt for “sharper” differentiationto nearby bands.

The next generation of mobile phones will be required to operate athigher frequencies to enable transmitting and receiving the ever growingdata traffic. Moving to such higher frequencies without enlarging themobile phone requires small low power resonators that operate at higherfrequencies and that can be used in smart phones without rapid depletionof the battery power pack.

The quality factor or Q factor is a dimensionless parameter thatdescribes how under-damped an oscillator or resonator is, andcharacterizes a resonator's bandwidth relative to its center frequency.The next generation of mobile phones requires quality resonators havinghigh Q factors.

Bulk-acoustic-wave (BAW) filters provide better performance than surfaceacoustic wave filters. Whereas the best SAW filters may have Q factorsof 1000 to 1500, current state of the art BAW resonators have Q factorsof 2500 to 5000.

BAW filters can operate at higher frequencies than SAW filters. Theyhave better power handling, a smaller size, higher electrostaticdischarge (ESD), better bulk radiation and less out of band ripple.

However, SAW filters are simpler and cheaper to manufacture and sincethe IDT pitch can be varied by the mask layout, resonators havingsignificantly different frequencies can be made on the same die, usingthe same piezoelectric film thickness.

The electrical impedance of a BAW resonator has two characteristicfrequencies: the resonance frequency f_(R) and anti-resonance frequencyf_(A). At f_(R), the electrical impedance is very small whereas atf_(A), the electrical impedance is very large. Filters are made bycombining several resonators. The shunt resonator is shifted infrequency with respect to the series resonator. When the resonancefrequency of the series resonator equals the anti-resonance frequency ofthe shunt resonator, the maximum signal is transmitted from the input tothe output of the device. At the anti-resonance frequency of the seriesresonator, the impedance between the input and output terminals is highand the filter transmission is blocked. At the resonance frequency ofthe shunt resonator, any current flowing into the filter section isshorted to ground by the low impedance of the shunt resonator so thatthe BAW filter also blocks signal transmission at this frequency. Thefrequency spacing between f_(R) and f_(A) determines the filterbandwidth.

For frequencies other than the resonance and anti-resonance frequencies,the BAW resonator behaves like a Metal-Insulator-Metal (MIM) capacitor.Consequently, far below and far above these resonances, the magnitude ofthe electrical impedance is proportional to 1/f where f is thefrequency. The frequency separation between f_(R) and f_(A) is a measureof the strength of the piezoelectric effect in the resonator that isknown as the effective coupling coefficient—represented by K² _(eff).Another way to describe the effective coupling coefficient is as ameasure of the efficiency of the conversion between electrical andmechanical energy by the resonator (or filter). It will be noted thatthe electromechanical coupling coefficient is a materials relatedproperty that defines the K² _(eff) for the piezoelectric film.

The level of performance of a filter is given by its factor of merit(FOM) which is defined as FOM=Q*K² _(eff).

For practical applications, both a sufficiently high K² _(eff) and highQ factor values are desired. However, there is a trade-off between theseparameters. Although K² _(eff) is not a function of frequency, theQ-value is frequency dependent and therefore the FOM (Factor of Merit)is also a function of frequency. Hence the FOM is more commonly used infilter design than in the resonator design.

Depending on the application, often device designers can tolerate alowering in the K² _(eff) to achieve a high Q factor where a smallsacrifice in K² _(eff) gives a large boost in the Q value. However, theopposite approach of sacrificing Q-value to obtain a design having anadequate K² _(eff) is not feasible.

K² _(eff) can be enhanced by choosing a high acoustic impedanceelectrode, and can also be traded off with other parameters such aselectrode thickness and a thicker passivation layer.

There are two main types of BAW resonators (and thus filters): SMR(solidly mounted resonators) and FBAR (Film Bulk Acoustic Resonatorresonators.

In the SMR resonator, a Bragg reflector is created under the bottomelectrode using a stack of alternating low and high impedance thin filmlayers, each having a thickness λ/4, where λ is the wavelength of thetarget frequency. The Bragg reflector stack acts an acoustic mirror toreflect the acoustic wave back into the resonator.

SMR resonators are easier (and thus typically cheaper) to manufacturethan FBAR resonators and since the piezoelectric film is attacheddirectly to the substrate, heat is dissipated more effectively. However,in SMR based filters, only the longitudinal acoustic wave is reflected,but not the shear waves. Consequently SMR filter designs have lower Qfactors than FBAR based filters.

In the FBAR resonator a free-standing bulk acoustic membrane which issupported only around its edge is used. An air cavity is providedbetween the bottom electrode and the carrier wafer. The high Q factor ofthe FBAR is a great advantage over the SMR.

The Commercial FBAR filter market is dominated by Broadcom™ (previouslyAVAGO™) which uses Aluminum Nitride (AlN) as the piezoelectric thin-filmmaterial that best balances performance, manufacturability and WaferLevel Packaging (WLP) processing that employs Si cavity micro-cappingover the FBAR device with TSV (through silicon via) for flip chipelectrical contacts. AlN has the highest acoustic velocity for apiezoelectric film (11,300 m/s) and hence requires a thicker film for agiven resonance frequency which eases process tolerances. Furthermore,high quality sputtered AlN films with FWHM (Full width at half maximumXRD peak) of less than 1.8 degrees allow K² _(eff) values that are above6.4% which is conveniently about twice the transmit band for FCCmandated PCS. With Q values reaching 5000, FOM values of 250 to 300 areachievable, representing best in class filter devices. K² _(eff) must bekept constant to meet the band requirement. Consequently, to improve theFOM of a filter generally requires increasing the Q value.

Despite the high performance of the above mentioned FBAR filters, issuesstill remain that prevent moving forward to the next generation ofwireless communication. The greater number of users sending andreceiving more data results in increasingly jammed bands. To overcomethis, future bandwidths should be more flexible to adapt to agilearrangements of different bands. For example, The 5 GHz WiFi band has 3sub-bands located at 5.150-5.350 GHz, 5.475-5.725 GHz, 5.725-5.825 GHz,respectively, corresponding to required K² _(eff) of around 7.6%, 8.8%and 3.4%. The coupling coefficient K² _(eff) is mainly decided by theintrinsic nature of the piezoelectric material, but is affected by thecrystalline quality and orientation of the piezo film, by exteriorcapacitors and inductors and by the thickness of the electrodes andother stacked materials. The bandwidth of AlN FBARs is mainly modulatedby inductors and capacitors that are pre-integrated into the ICsubstrate carriers. However, these elements degrade the Q factor andalso increase the substrate layer count and thus the size of the finalproduct. Another approach for K² _(eff) modulation is to use anelectrostrictive material to realize tunable band FBAR filters. Onecandidate material is Ba_(x)Sr_(1-x)TiO₃ (BST) that may be tuned oncethe DC electrical field is applied

Tunability with BST can also be achieved by using it as a variablecapacitor build in part with the FBAR resonators circuitry therebyassisting in matching filters and in adjusting their rejection.Furthermore, since a BST FBAR resonates only with a certain applied DCbias voltage, it may represent low leakage switching properties,potentially eliminating switches from the Front End Module (FEM) of themobile device and thereby simplifying module architecture and reduceboth size and cost. BST FBARs also possess other favorable propertiesfor RF applications. The high permittivity of ferroelectric materials(εr>100) allows for reduction in the size of devices; for example, atypical BST resonator area and BST filter area is in the order of 0.001mm² and 0.01 mm², respectively, at low GHz frequencies in standard 50-ΩRF systems. In fact, using BST the resonator size may be an order ofmagnitude smaller than that of conventional AlN resonators. Moreover,the power consumption in the BST FBAR itself is negligible even with theusage of the above-mentioned DC bias voltage across the device due to avery small leakage current in the BST thin-film.

Strong c-axis texture is the most important prerequisite for AlN or BSTbased FBARs because the acoustic mode for such FBARs needs to belongitudinally activated, and the piezoelectric axis of both AlN and BSTis along the c-axis. Hence high qualities single crystal piezo film, asrepresented by FWHM of less than 1°, have great impact on the FBARfilter properties and can reduce the RF power that is otherwise wastedas heat by as much as 50%. This power saving can significantly reducethe rate of drop calls and increase the battery life of mobile phones.

Epitaxial piezoelectric films with single orientation may have othermerits. For example, strongly textured epitaxially grown single crystalpiezo films are expected to have smoother surfaces than those ofrandomly oriented films. This in turn, results in reduced scatteringloss and a smoother interface between the metal electrodes to the piezofilms which both contribute to a higher Q-factor.

Furthermore, there is an inverse thickness to operating frequencyrelationship for AlN and BST filter films. Ultra thin-films are neededfor extremely high frequency filters such as 5 GHz WiFi, Ku and K bandfilters. For filter operating at 6.5 GHz the thickness of BST filmshould be around 270 nm and for 10 GHz the thickness of an AlN filmshould be around 200 nm. These dimensions invokes serious challenges forfilm growth because it is hard to attain the necessary stiffness for anextremely thin anchored membrane and the crystalline defects and strainsare more likely to cause cracks and mechanical failures as the membranefilm becomes thinner. As such, more innovative membrane supportingstructures with defect-free single crystal films are needed for the nextgeneration of high frequency FBARs.

Unfortunately, AlN, BST and other piezoelectric materials have vastlattice spacing and orientation differences to that of silicon and thoseof currently used bottom electrode metals. Furthermore, the range ofbottom electrode materials available, especially in the case of BST, isvery limited since they have to withstand relatively high temperaturesduring the subsequent deposition of the piezo film thereupon.

An alternate approach to PVD deposited AlN to achieve higher K² _(eff)and thus FOM, is exploring the usage of higher quality single crystalAlGaN using Chemical Vapor Deposition (CVD). High resistivity siliconsubstrates with <111> orientation can be used as substrates for thedeposition of such films thereon, and as is typical for III-N layergrowth on silicon, a thin AlN layer may be used as a buffer layer toaccommodate the large lattice mismatch between the substrate and theAlGaN film. Nevertheless, there is still a large difference in theCoefficient of Thermal Expansion (CTE) between AlGaN films and siliconwhich leads to the epitaxial layer being in tension at room temperatureand this residual stress may result in the film cracking.

U.S. Pat. No. 7,528,681 to Knollenberg titled acoustic devices using anAlGaN region, describes a method of creating a single crystal film ofAlGaN by epitaxially growing the thin film on a sapphire substrate and,after depositing a first electrode, the thin film is detached from thesubstrate using a laser lift-off process.

SUMMARY

An aspect of the invention is directed to a filter package comprising anarray of piezoelectric films sandwiched between lower electrodes and anarray of upper electrodes covered by an array of silicon membranes,which are themselves preferably single crystal silicon membranes withcavities thereover: the lower electrode being coupled to an interposerwith a first cavity between the lower electrodes and the interposer; thearray of silicon membranes, preferably constructed from single crystalsilicon having a known thickness and attached over the upper electrodeswith an array of upper cavities, each upper cavity between a siliconmembrane of the array and a common silicon cover; each upper cavityaligned with a piezoelectric film, an upper electrode and siliconmembrane, the upper cavities having side walls comprising SiO₂; theindividual piezoelectric films, their upper electrodes and siliconmembranes thereover being separated from adjacent piezoelectric films,upper electrodes and silicon membranes by a passivation material.

Typically the piezoelectric films comprises a material selected from thegroup consisting of Ba_(x)Sr_((1-x))TiO₃ (BST), AlN andAl_(x)Ga_((1-x))N.

Typically the piezoelectric films have a thickness of up to 2 microns.

Optionally the piezoelectric films each comprise a single crystal.

Typically the lower electrodes comprise aluminum.

In some embodiments edges of the lower electrodes are stiffened by anunder bump metallization material comprising a titanium adhesion layerfollowed by at least one of tungsten, tantalum and molybdenum.

In some embodiments an under bump metallization material comprisingtungsten or tantalum connects the lower electrode to the copper pillars.

Typically the lower electrode is coupled to the interposer by soldertipped copper pillars.

Optionally the upper electrodes comprise at least one metal layerselected from the group comprising aluminum, titanium, tungsten,molybdenum, gold and gold-indium layers.

In some embodiments the upper electrodes are multilayer electrodescomprise at least one layer with a relatively low DC resistance and asecond layer with a relatively high acoustic impedance.

Optionally the at least one layer with a relatively low DC resistancecomprises aluminum and is proximal to the piezoelectric layer, and thesecond layer with a high acoustic impedance comprises tungsten ormolybdenum that is further coupled to a bonding layer for attachment ofthe silicon membrane thereover.

In other embodiments, a single layer of tungsten, tantalum or molybdenumis deposited onto the piezoelectric layer on one side and a bondinglayer is applied on the opposite side for attachment of the siliconmembrane there-onto.

Optionally an adhesion layer is applied between at least one of thefollowing list of adjacent layers: the piezoelectric layer and therelatively low DC resistance layer; the relatively low DC resistancelayer and the relatively high acoustic impedance layer; the relativelyhigh acoustic impedance layer and the bond layer and between the bondlayer and the silicon membrane.

The adhesion layer is typically titanium.

In some embodiments the passivation material is selected from the groupconsisting of SiO₂, Si₃N₄, Ta₂O₅, polyimide and Benzocyclobutene (BCB).

Typically the upper cavities between the silicon membrane and thesilicon handle have side walls comprising residual silicon oxide.

Typically the cavities are 3-10 microns deep.

Optionally the upper electrodes of the array of upper electrodes areseparated from each other by a 3 to 10 micron wide band of thepassivation material.

In some embodiments there is a first adhesion layer between thepiezoelectric membrane and the lower electrode.

Typically the first adhesion layer comprises titanium.

Optionally the silicon wafer is attached to the upper electrode by anadhesion layer adjacent to the upper electrode, a bonding layer and afurther adhesion layer attached to the silicon wafer thereby creating acomposite electrode.

Typically the adhesion layer and further adhesion layer comprise Tihaving a thickness in the range of 5 nm to 50 nm with 5% tolerances.

The bonding layer is typically selected from the group comprising Au—Inalloy, Au and AlN.

Typically the silicon wafer has a thickness in the range of 1 micron to10 microns and is coupled to the silicon cover by silicon oxide.

Typically the silicon cover is 90 microns thick but could be as much as150 microns.

Typically the silicon cover over the cavity is perforated with throughsilicon via holes.

Typically the filter package is encapsulated in polymerover-mold/under-fill (MUF), wherein a barrier between the filter arrayand the interposer provides a perimeter wall of the lower cavity; thebarrier comprising at least one of an SU8 gasket around the filter arraythat is attached to the lower electrode and an epoxy dam attached to theinterposer.

Typically the interposer comprises at least one via layer and onerouting layer of copper encapsulated a dielectric matrix.

Optionally the interposer comprises a polymer matrix selected from thegroup consisting of polyimide, epoxy, BT (Bismaleimide/Triazine),Polyphenylene Ether (PPE), Polyphenylene Oxide (PPO) and their blends.

In some embodiments, the interposer further comprises glass fibersand/or ceramic fillers.

BRIEF DESCRIPTION OF FIGURES

For a better understanding of the invention and to show how it may becarried into effect, reference will now be made, purely by way ofexample, to the accompanying drawings.

With specific reference now to the drawings in detail, it is stressedthat the particulars shown are by way of example and for purposes ofillustrative discussion of the preferred embodiments of the presentinvention only, and are presented in the cause of providing what isbelieved to be the most useful and readily understood description of theprinciples and conceptual aspects of the invention. In this regard, noattempt is made to show structural details of the invention in moredetail than is necessary for a fundamental understanding of theinvention; the description taken with the drawings making apparent tothose skilled in the art how the several forms of the invention may beembodied in practice. In particular, it will be appreciated that theschematic illustrations are not to scale, and the thickness of some verythin layers is exaggerated. In the accompanying drawings:

FIG. 1 is a schematic not-to-scale cross section representation of aComposite FBAR filter module which combines a plurality of CompositeFBAR resonators coupled in half ladder or lattice arrangements orcombinations thereof.

FIG. 2 is a simplified circuit of a ladder type RF filter configuration;

FIG. 3 is a graph showing the transmission response of the ladder filterconfiguration of FIG. 2;

FIG. 4 is a is a simplified circuit of a lattice type RF filterconfiguration;

FIG. 5 is a graph showing the transmission response of the lattice typefilter configuration of FIG. 4;

FIG. 6 is a simplified circuit of a combined ladder and lattice type RFfilter configuration;

FIG. 7 is a graph showing the transmission response of the filterconfiguration of FIG. 6;

FIG. 8 is a flowchart illustrating a method of fabricating the CompositeFBAR structure of FIG. 1;

FIGS. 9 to 12 and 14 to 54 are schematic representations of the build upachieved by the steps of flowchart of FIGS. 8 and 9, and

FIG. 13 is a 180° XRD spectrum of single crystal Ba_(x)Sr_((1-x))TiO₃(BST).

DESCRIPTION OF EMBODIMENTS

By way of example, a design for a Composite FBAR filter module withsingle crystal Ba_(x)Sr_((1-x))TiO₃ (BST), AlN or Al_(x)Ga_((1-x))Npiezoelectric resonators is detailed hereunder with reference to FIG. 1,together with methods of manufacture with reference to FIG. 8.

With reference to FIG. 1 a Composite FBAR filter module 5 isschematically shown. The filter module 5 comprises a plurality ofcomposite FBAR resonators coupled in half ladder or lattice arrangementsor combination thereof. The composite FBAR resonators consist ofpiezoelectric films 18′, 18″ that may be Ba_(x)Sr_((1-x))TiO₃ (BST), AlNor Al_(x)Ga_((1-x))N separated by a passivation material 54 such asSiO₂, Si₃N₄, Ta₂O₅, polyimide and Benzocyclobutene (BCB) and sandwichedbetween electrode 22, 60.

In the method of construction described hereinbelow, it will be notedthat both top and bottom electrodes 22, 60 are deposited onto thepiezoelectric material 18 rather than by depositing a piezoelectricmaterial on top of an electrode which is currently standard practice forFBAR filter construction. This enables a wider range of metals such asaluminum to be used as the bottom electrode; aluminum having increasedconductivity and being less dense, enables decreasing the electrodeweight and the subsequent mechanical damping it causes to the resonator.The upper electrode may be a composite electrode comprising a number ofdifferent metal layers including the silicon film 30. However, theelectrode resonator material 18 is typically coupled to the silicon film30 that is preferably single crystal silicon by other layers of upperelectrode including a bonding layer (48, 50), adhesion layers 46, 46′and often by a relatively low DC resistance layer and a relatively highacoustic impedance layer.

Thus, in addition to the silicon membrane 30, the upper electrodetypically includes titanium adhesion layers, a relatively low DCresistance layer of aluminum, a relatively high acoustic impedance layerof tungsten or molybdenum and gold 50 or gold-indium 48 bonding layers.The silicon film 30 is typically a single crystal silicon and may haveany of the following orientations, <100>: <111> and <110>. It typicallyhas a thickness of 1 to 2 microns, but could be up to 10 microns thick.The single crystal silicon film 30 provides a mechanical support layerthat has low acoustic losses and is itself attached by a layer ofsilicon oxide 34 using SOI technology to a cover 32 that is a thickersilicon wafer and is also known as a ‘handle’, providing a ‘wafer onhandle’. Cavities 76 are provided within the silicon oxide layer 34opposite the piezoelectric resonator films 18. The depth of the cavitiesis typically 4 to 10 microns. The bonding between the electrode 22 andthe silicon film 30 may be achieved in a number of ways, such as by agold-indium eutectic 48, a gold layer 50 or an AlN layer 52.

A metallic adhesion layer becomes part of the upper electrode. Thinlayers of one of these bonding materials may be attached to both thepiezoelectric material 18 or first metallic layers thereupon, and to thesilicon film 30 by adhesion layers 46, 46′ such as titanium and then thethin layers of the bonding material are fused together.

The coated piezoelectric resonator array is attached to an interposer 85by interconnects comprising solder 68 capped copper pillars 66, and isencapsulated in a polymer underfill/over-mold 72. A gasket 70 isprovided around the filter, between the interposer 85 and the lowerelectrode 60 around the resonator array that defines the filter. Thegasket 70 may consist of SU-8 attached to the lower electrode 60 and anepoxy dam 86 may be built up from the interposer 85. The gasket 70 andepoxy dam 86 work together to prevent the underfill/over-mold 72 thatseals the unit from penetrating under the resonator array and define acavity 92 between the lower electrode 60 and the interposer 85.Additional cavities 76 are provided over the piezoelectric films 18′,18″ in the space between the silicon membrane 30 and cover 32, byselective removal of the silicon dioxide 34 by etching. A passivationmaterial 54 separates the upper electrode 22, adhesion layers 20, 46,46′, bonding layers 48/50/52 and silicon membrane 30 into separateregions supporting pairs of parallel resonators and separatingresonators that are connected in series.

An Under Bump Metallization (UBM) Layer 62 that comprises tungsten ortantalum or molybdenum (possibly with an adhesion layer of titanium),enables fabrication of the copper pillars 66 on the underside of thebottom electrode 60. Other remnants of the UBM 63 serve as stiffening“raised frame” around the perimeter of the lower electrode 60, which,being aluminum, has a very low weight. This “raised frame” structure isespecially useful in Composite FBARs as it helps minimize lateral-wavespurious modes that otherwise lower the Q factor of the device,regardless of the mode number. With such a raised frame, only the mainlateral mode is excited due to the new boundary conditions between theactive and outside region of the resonator membrane 18 that are createdby the raised frame 63. Additionally, with reference to FIGS. 2 to 7, itwill be noted that filters comprise shunt resonators and seriesresonators. The extra weight of the UBM 63 layer on the electrodecreates a mechanical damping effect that can assist in lowering theshunt resonator frequency response vs. the series resonator frequencyresponse and thus improve the overall performance of the filter.

The Commercial FBAR filter market is dominated by Broadcom™ which usesAluminum Nitride (AlN) as the piezoelectric thin-film material that bestbalances performance and manufacturability.

Embodiments of the technology disclosed herein below use AlN orAl_(x)Ga_((1-x))N or Ba_(x)Sr_((1-x))TiO₃ (BST), which is expected tohave fairly high Q and better K² _(eff) values.

Because the fabrication method allows single crystal piezoelectric filmsto be fabricated, improved factors of merit (FOM) are expected whencompared to the polycrystalline non-epitaxially grown films currently inuse.

With reference to FIG. 2, a simplified circuit of a half ladder typefilter configuration created by BAW resonators in series with shunt BAWresonators is shown. In a filter, resonators are combined in a ‘ladder’,wherein each ‘rung’ or ‘stage’ comprises two resonators: one in seriesand the other connected in shunt. With reference to FIG. 3, adding rungsto the ladder, improves the rejection of undesired frequencies, creatinga signal with less out-of-band rejection (a steeper skirt) but this isat the expense of insertion loss and greater power consumption. Withreference to FIG. 4, another resonator configuration may be a “lattice”,which, as shown in FIG. 5 has poorer cutoff but better out-of-bandattenuation.

With reference to FIG. 6, the ladder and lattice type circuits may becombined to provide the transmission response shown in FIG. 7. Thepossible arrangements of resonators to create filters is beyond thescope of this application, but methods for fabricating resonators thatare coupled in series and parallel are discussed hereunder withreference to FIGS. 42 and 43 and this enables arranging the resonatorsin the various ladder, lattice and combination arrangements.

Referring back to FIG. 1, in preferred embodiments, the resonator film18 is a single crystal piezoelectric of (BST Al_(x)Ga_((1-x))N) or AlN.

Ba_(x)Sr_((1-x))O₃ (BST) is tetragonal. <111> single crystal BST may bedeposited onto Al₂O₃ single crystal substrates.

AlN and AlGaN are HCP type Wurtzite crystal structures (C planeorientation). A strong C axis texture is the most important prerequisitefor AlN and AlGaN FBAR filters because the acoustic mode of the FBARneeds to be longitudinally activated and the piezoelectric axis of AlNand AlGaN is along c-axis. The addition of gallium to AlN makes iteasier to match lattice spacing of the film with that of the substrate.

Since there are no grain boundaries in a single crystal, the attenuationof the acoustic signal is minimal. This also minimizes the lost energythat is otherwise transferred into heat and which has to be dissipated.

Single crystal and strongly textured Ba_(x)Sr_((1-x))TiO₃ (BST), AlN andAlxGa(1-x)N films have smoother surfaces than randomly oriented films.This results in reduced scattering loss and higher Q-factors.Furthermore, rough surfaces, especially at high frequencies, are a majorcause of the loss of the metal electrodes interfaces because of a skineffect. The smooth electrode—piezolelectric interfaces obtainable inhighly textured and single crystal films with both upper and lowerelectrodes deposited thereupon are thus extremely advantageous.

Composite FBAR structures consist of a thin piezoelectric film 18sandwiched between top and bottom electrodes 22, 60. In the past, theelectrode 22 was first deposited and then the piezoelectric layer 18 wasfabricated thereupon. This required the electrode 22 to be made from aheavy metal such as platinum, molybdenum, tungsten or gold, which allowthe high deposition temperatures required for subsequent piezoelectricfilm deposition thereupon. However, most of these metals have poor DCresistance, potentially deteriorating the Q factor of the resonator. Inpreferred embodiments described herein the electrodes 22, 60 aredeposited onto the piezoelectric film 18 using physical vaportechniques. This enables lightweight metals such as aluminum to be used,either on its own or in conjunction with other metal layers to formcomposite electrodes. Aluminum has a high conductivity and so a thinnerelectrode is possible. Aluminum is much less dense than refractorymetals and so the weight of the electrodes and their damping effect isless. The quality and coupling of the resonators and filters thus formedare vastly superior to those of the prior art.

Aluminum is expected to readily adhere to AlN and AlxGa(1-x)N due to theAl ions of the piezoelectric film. If, however, electrode adhesionproves difficult, an adhesion layer which adheres to both the singlecrystal and to the electrode may be used. For example, titanium may beappropriate. Such adhesion layers typically have thickness of tens ofnanometers.

The mechanism used in ferroelectric Composite FBAR transducers iselectrostriction which is the electric field induced piezoelectriceffect. The top and bottom electrodes 22, 60 are used to apply directcurrent (DC) and radio frequency (RF) signals. The preferred CompositeFBAR Composite structure described herein consists of a thin film singlecrystal Ba_(x)Sr_((1-x))TiO₃ (BST), AlN and Al_(x)Ga_((1-x))N film 18sandwiched between top and bottom aluminum electrodes 22, 60. The AlN orAl_(x)Ga_((1-x))N film 18 converts mechanical to electrical energy andvice versa.

To provide stiffening without substantial weight, a low acoustic-losssingle crystal silicon membrane layer 30 with possible orientation of<111>, <100> or <110> may be coupled to the piezoelectric films 18. Thesilicon layer 30 may have a thickness in the range of 1 μm to 10 μm,with the lowest possible thickness being preferable for best performancehigh frequency resonators. It should be noted that in Composite FBARsthere are odd and even resonance modes, where each mode exhibits peak Qand K² _(eff) Coupling as a function of the Ba_(x)Sr_((1-x))TiO₃ (BST),AlN and Al_(x)Ga_((1-x))N to silicon membrane thickness ratio. The peakK² _(eff) values decrease with mode number because the fraction ofacoustic displacement across the Ba_(x)Sr_((1-x))TiO₃ (BST), AlN andAl_(x)Ga_((1-x))N is reduced. However, the peak Q factor values increasewith mode number, since the fraction of acoustic displacement across thelow loss silicon layer increases. Hence, careful selection of theresonance mode is required for optimal FOM and low thickness siliconmembranes with low thickness BaxSr(1-x)TiO3 (BST), AlN andAl_(x)Ga_((1-x))N films are desired for higher frequencies filters. Inshould also be noted that higher harmonic modes naturally render ahigher frequency for a given film thickness and this may alleviate therequirement for trimming. Consequently, operating an FBAR at its secondor higher harmonic mode frequency can extend the FBAR operationfrequency range, as long as its figure of merit (FOM) remains high.Cavities 76, 92 are provided above and below the piezoelectric 18 onsilicon 30 combination. The structure is encapsulated with a polymer 72and mounted on an interposer 85 and coupled thereto with copper pillars66 that are typically about 40-50 μm wide and about 40 μm high andjoined to upper contact pads 82 of the interposer 85 with solder 68. Apolymer gasket 70 which may be fabricated from SU-8 to have a high formfactor and/or a dam 86 (typically epoxy) may be provided around theperimeter of the filter structure to keep the polymerover-mold/under-fill (MUF) 72 from entering the lower cavity 92. Theinterposer 85 may be constructed using well established fabricationtechnologies.

The Composite FBAR shown in FIG. 1 has such a piezoelectric 18 onsilicon membrane 30 Composite FBAR structure, preferably wherein thepiezoelectric film 18 is a Ba_(x)Sr_((1-x))TiO₃ (BST), AlNAl_(x)Ga_((1-x))N and the electrodes 22, 60 are fabricated fromlightweight aluminum.

Although RF resonators are primarily used as filters, they find otheruses, including as sensors, for example. There is also interest intunable resonators that can operate at different frequencies.

FIG. 8 shows the steps of the main process flow for fabricating anComposite FBAR filter, and schematic illustrations of the build-up areshown in FIGS. 9-12, 14-53.

With reference to FIGS. 8 and 9, the method consists of first obtainingand providing a removable wafer carrier with a release layer—step (a), aschematic, not to scale representation of this is shown in FIG. 9 whichrepresents a c-axis <0001>±1° Sapphire wafer 10 with an un-doped GalliumNitride (U-GaN) release layer C-axis <0001>±1° 12 thereon. Such sapphirewafers 10 with U-GaN 12 deposited thereon are commercially availablewith diameters of 2″, 4″ and 6″ in thicknesses of from 430 μm, and havea polished surface with an RMS smoothness of less than 1 nm. The U-GaNlayer 12 typically has a thickness of 4 μm and a polished surface havingan RMS of less than 1 nm ready for epitaxial growth thereon. Thesecoated substrates were developed for the Light Emitting Diode (LED)industry and are commercially available from various Chinesemanufacturers including San'an Optoelectronics Co., Ltd. (San'an) andSuzhou Nanowin Science and Technology Co., Ltd (NANOWIN)™.

Alternatively, an AlN single crystal wafer cleaved from a large singlecrystal such as a single crystal grown by the Czochralski method, andhaving an appropriate laser absorbing release film thereupon could beused.

A piezoelectric film comprising BaxSr(1-x)TiO3 (BST), Al_(x)Ga_((1-x))Nor AlN is now deposited onto the removable carrier 10—step (b). Withreference to FIG. 10, to aid heat dissipation and thus thicknessdistribution during subsequent deposition of a Ba_(x)Sr_((1-x))TiO₃(BST), Al_(x)Ga_((1-x))N or AlN piezoelectric film, a metal layer 14 maydeposited on the back of the sapphire wafer 10—step (bi), i.e. the sideopposite to the side coated with GaN 12. The thickness of the metallayer 14 depends on the metal used. In this instance, withBa_(x)Sr_((1-x))TiO₃ (BST), Al_(x)Ga_((1-x))N or AlN piezoelectricmaterial 18 subsequently deposited (see below) titanium is a goodcandidate for the heat dissipating metal layer 14, and an appropriatethickness for the heat dissipating layer 14 is about 150 nm. The heatdissipating metal layer 14 may be deposited by sputtering, for example.

The Gallium Nitride release layer 12 is typically about 4 nm thick withan RMS roughness of less than 2 nm. Because of the lattice matchingbetween the <0001> plane of the GaN 12 and Sapphire 10,Ba_(x)Sr_((1-x))TiO₃ (BST), Al_(x)Ga_((1-x))N or AlN may be laid down asa single crystal film. Adjusting the percentage of gallium helps ensurelattice matching and thin films having a thickness of between 100 nm and2000 nm and typically 200-400 nm in the case of Ba_(x)Sr_((1-x))TiO₃ and200 nm to 2000 nm in the case of AlN or Al_(x)Ga_((1-x))N are thendeposited in this manner using oxide molecular beam epitaxy (MBE) usingeffusion cells of barium, strontium and titanium or Al and Ga—step (b).

Molecular beam epitaxy (MBE) is a high purity low energy depositiontechnique that allows for low point defect manufacturing. It is possibleto control the barium to strontium (or aluminum to gallium) ratio withhigh accuracy of ±1% and this affects the Q factor and coupling of thefilm.

The epitaxially grown Ba_(x)Sr_((1-x))TiO₃ (BST), Al_(x)Ga_((1-x))N orAlN films may have a RMS roughness of less than 1.5 nm. This minimizesthe so called ripple effect.

As shown in FIG. 12, to facilitate deposition of a single crystalpiezoelectric layer 18 of Ba_(x)Sr_((1-x))TiO₃, a buffer layer 16 ofrutile TiO₂ and/or SrTiO₃ may first be deposited step b(ii).

To the best of our knowledge, Applicant is the first person to create asingle crystal layer of Ba_(x)Sr_((1-x))TiO₃ and FIG. 13 is a 180° XRDspectrum of the structure of FIG. 12 showing that a single crystalmembrane of BST was obtained.

However, It will be noted that commercially available Al_(x)Ga_((1-x))Nthin films with x of 5, 10, 13, 20, 30, 50 and 100% and XRD FWHM of afraction of a degree are available from various sources. For examplesingle crystal Al0.2Ga0.8N with a c plane orientation <0002> having athickness of 1 or 2 μm deposited on a 4 μm GaN release layer on a 4″sapphire substrate is available from Kyma Technologies™.

In prior art resonators, the lower electrode is first deposited and thenthe piezoelectric film is deposited thereon. Consequently, due to thehigh temperature fabrication of the piezoelectric film, refractorymetals such as molybdenum, tungsten, platinum or gold are traditionallyused for the lower electrode

Since in the present technology, the first electrode 22 is depositedonto the piezoelectric film, a wide range of metals may be used such asaluminum. It will be appreciated that aluminum has a relatively low DCresistance when compared to these refractory metals, and thus usingaluminum electrodes is expected to increase the Q factor of the filter.

FIG. 14 shows the equivalent structure with a piezoelectric film 18 ofAl_(x)Ga_((1-x))N or AlN without a buffer layer 16. For simplicity, theremaining structures shown in FIGS. 15 to 53 do not show the bufferlayer 18.

A first electrode layer 22 is now deposited over the piezoelectricmembrane 10—step (c). With reference to FIG. 16, to aid adhesion, anadhesion layer 20 such as a titanium layer that may be as little as 5 nmthick, but could be as much as 50 nm is first deposited onto thepiezoelectric membrane 18—step (ci). Then an aluminum electrode layer 22having a thickness of, say, 50 nm to 150 nm is deposited thereover—step(cii). Both the adhesion layer 20 and the electrode layer 22 may bedeposited by sputtering, for example. Tolerances of ±5% are acceptableand easily obtainable.

At a first approximation, the resonant frequency f_(R) of apiezoelectric resonator is given by the following equation:f_(R)=υ/λ≈υL/2t where υL is the longitudinal acoustic velocity in thenormal direction of the piezoelectric layer, t is the thickness of thepiezoelectric film and λ is the acoustic wavelength of the longitudinalwave.

However, in practice, the acoustic properties of the other layers of theresonator affect the resonator performance. In particular, the massloading effect of the electrodes which tend to be made of heavy metalssuch as molybdenum and platinum, due to the need to withstand thefabrication temperature of the piezoelectric material.

Although described for depositing aluminum onto Ba_(x)Sr_((1-x))TiO₃,Al_(x)Ga_((1-x))N or AlN, it will be appreciated that PVD or CVD withotherwise, low density, high conductivity electrode materials 22 overdifferent piezo layers may be used with the same method. For example,carbon nano-tubes (CNT) over single crystal AlN or AlxGa(1-x)N may beconsidered. Aluminum is particularly attractive for resonator electrodessince it has high electrical and thermal conductivity and a low density,so hardly lowers the overall Q factor of the resonator. However,previous manufacturing routes wherein the electrode was deposited priorto deposition of the piezoelectric, ruled out aluminum. In this regardit will be noted that adding an aluminum bottom electrode after etchingaway a Si carrier wafer and exposing the back side of the piezoelectriclayer, has significant yield challenges and complicates the packagingprocess of the filter and thus lowers the final yield.

The piezoelectric film 18, adhesion layer 20 and aluminum electrode 22are deposited over the entire sapphire wafer 10 as a continuous layer.

A backing film on handle 28 is obtained—Step (d). This is a commerciallyavailable silicon on insulator (SOI) product. The backing film on handle28 is typically a silicon wafer film 30 sandwiched to a silicon carrier32 by a silicon oxide layer 34.

A commercially available backing film on handle 28 obtainable from KSTWorld Corp™ (www.kstworld.co.jp) or OKMETIC™ (www.okmetic.com) that issuitable is shown schematically in FIG. 17 and consists of a siliconfilm 30 that comes in thicknesses in the typical range 1 to 10 μm thatis coupled by a SiO2 box 34 that is typically 3-10 μm thick to a Siliconhandle 32 that is typically at least 400 μm thick.

An alternative SOI product 36 shown in FIG. 18 is a silicon wafer 38attached to a silicon carrier 42 by a silicon oxide layer 40, but with apreformed air cavity 44. Such a structure is commercially available fromIcemos™ (www.icemostech.com).

Both SOI products 28, 36 may be obtained pre-coated with metal coatingson the silicon film 30, 38 aiding their attachment to the piezoelectricfilm—electrode sandwich.

With reference to FIG. 19, the commercially available film 30 (38) onhandle 32 (42) product 28 (36) is attached to the electrode 22 of thestack—step (e).

There are a number of ways that the silicon film 30 (38) may be attachedto the electrode 22. For example, with reference to FIG. 20 an adhesionlayer such as titanium 46 may be deposited onto the electrode layer 22and this can be coated with an adhesive layer consisting of agold-indium eutectic alloy 48 comprising 0.6% gold and 99.4% indium. TheAu—In eutectic melts at 156° C. and by hot pressing at about 200° C.,the adhesion layer may be attached to the silicon membrane 30 of the SOIwafer 28. Optionally, a titanium bonding layer 46′ is attached to thesilicon membrane 30 and an adhesive layer of gold-indium eutectic alloy48 is attached to this. The two adhesion layers are fused together bythe hot processing. The bonding layers created by this technique arerather thick, and the process is capable of some variation. Withreference to FIG. 21, an alternative process relies on the fact thatboth the exposed surface of the silicon wafer film 30 (38) and thesurface of the electrode layer 22 are very smooth. By coating bothsurfaces with adhesion layers of titanium 46, 46′ that are typically 2-4nm thick and may be deposited by sputtering, and then depositing puregold 50 (50′) coatings of thicknesses of 10-40 nm onto the adhesionlayers 46 (46′) the two gold coatings 50, 50′ may be brought together atroom temperature and the coatings fused together (see for exampleShimatsu, T. & Uomoto, M. (2010). “Atomic diffusion bonding of waferswith thin nanocrystalline metal films”. Journal of Vacuum ScienceTechnology B: Microelectronics and Nanometer Structures. 28 (4). pp.706-714.). This technique requires a lower temperature and a thinnergold layer 50 than the Au—In layer shown in FIG. 20.

With reference to FIG. 22, a further alternative process is to againcoat both surfaces with adhesion layers of titanium 46, 46′ that aretypically 2-4 nm thick, and then deposit aluminum nitride 52 (52′)coatings having thicknesses of 10-40 nm onto the adhesion layers 46(46′). The two aluminum nitride 52 (52′) coatings may be activated withAr plasma and when brought into contact at room temperature andpressure, fuse together. The bond can be strengthened by annealing at300° C. in a N2 atmosphere, typically for a period of 3 hr.

It will be appreciated that the stack of titanium adhesion layers 20,46, 46′ and the gold-indium or gold bonding layers 48, 50 serve with thealuminum electrode 22 layer as the upper electrode. This compositeelectrode can take advantage of the inherent characteristics such as DCresistance, acoustic impedance and weight (density) of the differentmaterials, to provide different properties to the composite electrode.

An alternative composite electrode may include an aluminum lower layerand gold-indium/gold/AlN bonding layers with an intermediate doublelayer of titanium and tungsten or titanium and molybdenum between thealuminum and the bonding layer. Titanium, tungsten and molybdenum mayall be deposited by sputtering, and the titanium layer serves as anadhesion layer. The addition of a tungsten or molybdenum layer not onlyincreases the acoustic impedance but additionally serves as a barrierlayer between the gold of the bonding layer and the aluminum layer. Insuch a structure, the thickness of the aluminum layer may be as littleas 50 nm. The titanium-tungsten or titanium-molybdenum is typically also50 nm or slightly thicker. In such structures the gold bonding layer maybe reduced to the minimum thickness that allows bonding while thealuminum, titanium-tantalum, titanium-tungsten or titanium-molybdenumserve as the main metals of the composite electrode, since they providea desirable balance of low DC resistivity with high acoustic impedance.

In general, it is advisable to process at as low a temperature aspossible to minimize the likelihood of damage to the piezoelectric filmand its electrodes and to further minimize warpage of the stack due todifferences in the coefficient of thermal expansion of silicon andsapphire. It is further advised that the bonding layer thickness shouldbe as thin as possible in order to enhance the Q factor value but thathigher bonding layer thicknesses are also possible thorough carefulbalancing of the DC resistance, weight and acoustic impedances of thecomposite electrode.

Once the silicon film and handle 28 is attached, the sapphire substrate10 may be removed—step (f). If a thermal layer such as titanium 14 isdeposited on the back of the substrate, this may be removed by chemicalmechanical polishing, for example, giving the structure shownschematically in FIG. 23.

Then, the GaN 12 may be irradiated through the sapphire substrate 10using a 248 nm excimer laser to disassociate the GaN 12 enabling liftoff of the sapphire substrate 10. Such a pulsed laser, with a squarewaveform is available from IPG Photonics™. This process is known aslaser lift off and results in the structure shown schematically in FIG.24.

Residual GaN 12 may be removed using Inductively Coupled Plasma (ICP)with Cl₂, BCl₃ and Ar for example—FIG. 25. This can be achieved attemperatures of below 150° C., avoiding heat treatment of thepiezoelectric thin film 18 and of the aluminum 22 and other layers. TheInductively Coupled Plasma (ICP) is a commercially available process,used by NMC (North Microelectrics) China Tool and by SAMCO INC™, forexample.

After removing the GaN 12 a thickness measurement and trimming processof the piezoelectric film 18 may be required to obtain perfect frequencyresponse which is related to the film thickness—step (g). The trimmingprocess uses Ar+ Ion beam milling and this process may be used to tailorany metal adhesion, barrier or oxide layers such as SiO₂, Al₂O₃, AlN, W,Mo, Ta, Al, Cu, Ru, Ni or Fe where the wafers is held in a 4 axis chuckand rotated accordingly. A commercially available system known asInoScan™ is available from Meyer Burger™, Germany. A trimmedpiezoelectric layer 18 is shown in FIG. 26 which is flipped over. Itshould be noted that in order to obtain high performance RF filters witha high yield, the thickness of the piezoelectric layer may need to betrimmed to tenths of a nanometer (single digit angstroms).

The same ICP process that is used to clean the back side of thepiezoelectric 18 may then be used to pattern the piezoelectric layer 18into arrays of piezoelectric islands for fabricating filters and thelike—step (h). By way of example only, a schematic top view is shown inFIG. 27 and a side view in FIG. 28. Although rectangular islands ofpiezoelectric are shown, the islands may, of course, have any shape asdictated by the shape of the lithography mask tool. It will be notedthat patterning the piezoelectric layer 18 into separate membranes justafter the laser lift off processing reduces the risk of thepiezoelectric layer cracking due to stress release across the wafer.

An induction coupled plasma (ICP) using Cl₂+BCl₃+Ar, CF₄+O₂ or Cl₂+O₂+Arand SF₆+O₂ is then applied to respectively remove the aluminum, adhesionlayers, the high acoustic impedance layer, bonding layer and siliconmembrane 30 down to expose the top surface of the silicon oxide 34creating trenches 21—step (i). A top view of the structure is shown inFIG. 29 and a side view in FIG. 30. With reference to FIG. 54, usefully,the high acoustic impedance layer 22, bonding layer 48/50/52, adhesionlayers 46 are traversed by the trench 21 created by the ICP whichpenetrates into the silicon membrane 30. Then the trench 21 isre-patterned with photoresist and a narrower trench 27 is etched throughthe rest of the silicon layer 30 into the silicon dioxide 34. Thiscreates a stepped interface between the silicon membrane 30 and thesubsequently deposited passivation layer 54 (see below) and enablessecure anchoring of the passivation layer 54 to the silicon membrane 30in step (j) since the surface of the interface can still be severalmicrons, even if the silicon membrane 30 itself is only a micron or sothick. The silicon membrane 30 has to be fully traversed since siliconis a conductor, albeit not a particularly good one. The narrower trench27 that separates the individual silicon membranes is typically 3 to 10micron wide so that the individual silicon membranes are insulated fromeach other by a barrier of the passivation material 54 that is 3 to 10microns wide.

The depth of the cavities is typically 3-10 microns as well.

The induction coupled plasma (ICP) process operates at a temperature ofless than 150° C. and does not adversely affect the piezoelectricmembranes 18′, 18″ which are protected by the photo-resist mask.Inductively Coupled Plasma (ICP) is a commercially available process,used by NMC (Beijing North Microelectronics) China Tool and by SAMCOINC™, for example.

A schematic top view of the resulting structure is shown in FIG. 29 anda schematic side view is shown in FIG. 30.

A passivation layer 54 such as SiO₂, Si₃N₄, or a photo-sensitivePolyimide or BCB (Benzocyclobutene) is applied into the trenches 21 thusproduced—step (j). The same passivation material 54 may be used to coverthe piezoelectric islands 18′, 18″ with windows then being opened downthrough the passivation layer to the piezoelectric islands. Where aphotosensitive polyimide or BCB is used, this is achieved by selectiveexposure, which is a precision process that includes the known series ofsub-processes such as spin-coat, exposure, development and cure ofphoto-sensitive polymer passivation layers. Photo-sensitive polyimidepassivation materials are available from HD Microsystems™ and are astandard industry solution for Flip Chip and Wafer Level Chip ScalePackages (WL-CSP) devices such as that described in this specification.Photo-sensitive BCB is commercially available as Cyclotene™ from DowChemicals™.

SiO₂ and Si₃N₄ may be deposited using PE-CVD processes as known.

A schematic top view of the resulting structure is shown in FIG. 31 anda schematic side view is shown in FIG. 32.

The upper electrodes are now applied—step (k). An adhesion layer 58 suchas titanium is first deposited—step 3 (ki)—FIG. 33, and then the topelectrode 60 is then deposited—step (kii)—FIG. 34. Both the adhesionlayer 58 and the electrode 60 may be deposited by sputtering, forexample. Tolerances of ±5% are acceptable and easily obtainable.

Couplings are now applied to connect the structure to an interposer,described below. Firstly, an Under Bump Metallization (UBM) layer 62 maynow be applied—step (l) by depositing a layer of metal that may be Ti/W,Ti/Ta or Ti/Mo (typically about 25 nm titanium, followed by about 50 nmof tungsten, tantalum or molybdenum—step (li), FIG. 35. Sputtering maybe used.

The structure may then be covered with a layer of copper 64 that istypically about 1 μm thick, by sputtering, for example—step (lii)—seeFIG. 36; the Under Bump Metallization layer 62 keeps the copper 64 andaluminum 60 separate.

Next, copper pillars 66 may be fabricated—step (liii), FIG. 37. Theseare typically about 40-50 μm in diameter and about 40 μm high. They maybe fabricated by depositing a layer of photoresist 65, patterning andthen electroplating copper 66 into the pattern.

With reference to FIG. 38, solder 68 may then be deposited into thepattern to cap the copper pillars 66—step (liv). This could be achievedby electroplating or electro-less plating a suitable material into thephotoresist pattern used for fabricating the copper pillars 66. Then thephotoresist is stripped away—step (lv), FIG. 39.

The copper layer 64 around the copper pillars 66 is now etched away—step(lvi), FIG. 40. This may be accomplished by exposing to a solution ofammonium hydroxide at an elevated temperature. Alternatively, copperchloride or other commercially available Cu micro-etch solution may beused as the etchant. The UBM 62 is now selectively removed—step (vii),FIG. 41, leaving perimeter sections 63 over what will become the edgesof the upper electrode to add weight to the edges of the effectiveresonators. Such “raised frame” structure is especially effective inComposite FBARs to help minimize lateral-wave spurious modes thatotherwise lower the Q factor of the device, regardless of the modenumber. With such structures, only the main lateral mode is excited dueto the new boundary conditions created by the raised frame between theactive and outside region of the resonator membrane 18. Additionally,the extra load of the UBM 63 layer over the resonators provides adamping effect that can assist in lowering the shunt resonator frequencyresponse vs. the series resonator frequency response and thus improvethe overall performance of the filter.

With reference to FIG. 42, by way of schematic illustration only, topand side views of a pair of piezoelectric capacitors coupled in parallelis shown, and with reference to FIG. 43, by way of schematicillustration only, top and side views of a pair of piezoelectriccapacitors coupled in series is shown. The superfluous aluminum 60beyond that required for the electrode may be selectively removed byapplying an inductively coupled plasma comprising Cl₂+BCl₃+Ar and theexcess parts of the titanium adhesion layer 58 thereby exposed may beselectively removed by reactive induction etching away with CF₄ and O₂.

It will be noted that the extra weight of the UBM 63 layer providesmechanical damping that can lower the shunt resonator frequency responsevs. the series resonator frequency response, and thus improve theoverall performance of the filter.

With reference to FIG. 44, a polymer gasket 70 may now be fabricatedaround an array or resonators defining a filter—Step (n). This may beachieved using SU-8 technology. SU-8 is a commonly used epoxy-basednegative photoresist whereby the parts exposed to UV becomecross-linked, while the remainder of the film remains soluble and can bewashed away during development. SU8 can be deposited as a viscouspolymer that can be spun or spread over a thickness ranging from below 1μm to beyond 300 μm. It is an attractive material since it can bedeposited as a tall thin wall that can be about 55 μm high and thuscompatible with the solder capped copper pillars, whilst having a widthof from 10 to 30 μm.

At this stage, as shown in FIG. 45, the array of filters may be attachedto a tape 72 with the copper pillars 66 and SU8 gasket 70 side facingdownwards, and the silicon handle 32 may be thinned down to about 90microns—step (o), using chemical mechanical polishing (CMP), to producethe structure shown in FIG. 46. Other possible thinning techniquesinclude mechanical grinding, chemical polishing, wet etching togetherwith atmospheric downstream plasma (ADP) and dry chemical etching (DCE),for example,

Unless a SOI substrate 36 having prefabricated cavities 44—FIG. 18—wasused, cavities 76 are now formed in the SiO₂ 34 layer—step (p). Throughsilicon via etching (TSV) is used to drill holes 74 through the thinneddown silicon handle 32 to the SiO₂ box 34—step p(i), FIG. 47 oppositeeach of the piezoelectric films 18′, 18″.

The Silicon Oxide 34 may then be selectively etched away with HF vaporin accordance with the formula SiO₂+4 HF(g)→SiF₄(g)+H₂O through thesilicon via holes 74 to form cavities 76,—step p(ii), FIG. 48. Dry vaporetching is preferable to a wet etch since this enables penetration ofsmall features and prevents the membrane and cover from stickingtogether.

Up until this stage, the filters are fabricated in arrays using on waferfabrication techniques. The array is now diced into separate filterunits—step (q).

Dicing may take place by mechanical blades, plasma or laser. Plasma orlaser may be preferred with some designs in order to avoid membranedamages.

Such dicing tools are available by Disco™ Japan.

An interposer 85 is now procured step (r). By way of enablement only, atwo layer interposer 80 may be fabricated by copper electroplating ofpads 80 and vias 82 into photoresist on a sacrificial copper substrate,followed by laminating with a dielectric material 84 having a polymermatrix such as polyimide, epoxy or BT (Bismaleimide/Triazine),Polyphenylene Ether (PPE), Polyphenylene Oxide (PPO) or their blends,either provided as a film, or as a pre-preg reinforced with glass fibersfor additional stiffness. More details may be found in U.S. Pat. No.7,682,972 to Hurwitz et al. titled “Advanced multilayer corelessstructures and method for their fabrication” incorporated herein byreference. There are, however, alternative established manufacturingroutes for fabricating appropriate interposers. An appropriateinterposer 85 with copper pads 80 and vias 82 in a dielectric withpolymer matrix 84 is shown in FIG. 49.

In general, the interposer 85 should be thin so that the overall packageremains thin. However, it will be appreciated that different resonators18′, 18″ may be interconnected via routing layers within the interposer85, and additional layers may be built up if required.

With reference to FIG. 50, usefully an epoxy dam structure 86 may firstbe deposited on the interposer surface—step (s). The epoxy dam structure86 may be fabricated by silk-screening an epoxy polymer, or bylaminating a dry-film epoxy dam barrier that is photo-imageable. Thelast method is preferred as it provides high position accuracy withrespect to the SU8 gasket 70 on the filter die. It should be noted thatdry films may be deposited in several layers to achieve desiredthicknesses. As with the gasket 70 around each filter array, the dam 86could also be fabricated from SU-8. The dam 86 is designed to fit aroundthe gasket 70 and could be slightly larger or smaller in area than thearea surrounded by the gasket 70 to be positioned on the inside oroutside of the gasket 70. Indeed two dams 86 (one encircling and theencircled by the gasket) or a plurality of gaskets 70 could be provided.

As shown in FIG. 51, the interposer may then be attached to theComposite FBAR resonator array by aligning and melting the solder caps68 on the copper pillars 66—step (t).

As shown in FIG. 52, the array device may be encapsulated in polymer90—step (u); the dams 86 and SU8 gaskets 70 working together preventingunder fill of the cavity 92 within the gasket 70.

In this manner, the closely aligned SU8 connected to the Composite FBARarray and the epoxy dam connected to the substrate prevents under-fill72 from filling the cavity 92 under the piezo resonators 18′, 18″.

The array of resonators is then diced into separate filter modules—stage(v), giving the structure shown in FIG. 1, for testing, packaging andshipment.

The interposer 85 may be a functional substrate with embedded inductors,lines and couplers. It should be noted the interposer 85 maysubsequently be placed on the same IC Substrate together withcontrollers, power amplifiers and switches to generate a fullyintegrated Front End Module (FEM). This allows all components to bedesigned together to achieve optimum system performance. Instead of apolymer based interposer, an interface with a Low Temp Co-fired Ceramic(LTCC) may be used.

Thus single crystal Composite FBARs are shown and described.

In resonator/filter designs with a BaxSr(1-x)TiO3 piezoelectric layer,the top electrode may be split into two sections: the Al electrodeitself and a separated Al line that runs bias voltage to thepiezoelectric membrane and causes it to resonate. This bias voltage isusually between 5V to 40V, the voltage depending on the resonatorfrequency. For example, Tests performed on 2700 Å Piezo thick BST at 19Vhave caused the BST to resonate at 6.5 GHz.

Single crystal Ba_(x)Sr_((1-x))TiO₃ Filters are potentially tunableusing capacitors build around the filter on the same silicon carrier. Ithas been established by numerous research groups that single crystal BSThas a tunability ratio of 1:8 or even 1:10 whereas amorphous orpolycrystalline BST has only has 1:3 to 1:4 tunability.

Single crystal BST, AlN and AlGaN FBAR resonators and thus filters havethe following advantages:

-   -   Such filters may save up to half of the RF power wasted as heat        in prior art filters because the single crystal orientation        enables polarization of the excited acoustic wave.    -   The filters disclosed herein may operate at higher frequencies        since the thickness of the ultra-thin piezoelectric membrane        necessary for high frequencies is supported by an additional        silicon membrane (composite FBAR).    -   Having a composite electrode and structure that includes a        silicon membrane, such filters may have second or higher        harmonic mode frequencies that can extend the operating        frequency range of the FBAR    -   Single crystal BST, AlN and AlGaN FBARs disclosed herein use        well-known MEMS and LED FAB manufacturing processes rather than        dedicated and expensive Si FABs. This may simplify and reduce        the investment and total cost to manufacture the filter device.    -   Single crystal FBARs manufacturing processes disclosed herein        use the low cost back-end processes well established and with        high yields available by multiple wafer bumping and assembly        houses.

Although discussed hereinabove with reference to communication filters,it will be appreciated that thickness-shear-based Composite FBARs andsurface generated acoustic wave—based Composite FBARs are also used inother applications. For example they are widely used in biosensors sincethey provide high sensitivity for the detection of biomolecules inliquids.

Thus persons skilled in the art will appreciate that the presentinvention is not limited to what has been particularly shown anddescribed hereinabove. Rather the scope of the present invention isdefined by the appended claims and includes both combinations and subcombinations of the various features described hereinabove as well asvariations and modifications thereof, which would occur to personsskilled in the art upon reading the foregoing description.

In the claims, the word “comprise”, and variations thereof such as“comprises”, “comprising” and the like indicate that the componentslisted are included, but not generally to the exclusion of othercomponents.

The invention claimed is:
 1. A filter package comprising an array ofpiezoelectric films sandwiched between lower electrodes and an array ofupper electrodes comprising metal layers and silicon membranes withcavities thereover: the lower electrode being coupled to an interposerwith a first cavity between the lower electrodes and the interposer; thearray of silicon membranes having a known thickness and attached overthe upper electrodes with an array of upper cavities, each upper cavitybetween a silicon membrane of the array and a common silicon cover; eachupper cavity aligned with a piezoelectric film, an upper electrode andsilicon membrane, the upper cavities having side walls comprising SiO₂;the individual piezoelectric films, their upper electrodes and siliconmembranes thereover being separated from adjacent piezoelectric films,upper electrodes and silicon membranes by a passivation material.
 2. Thefilter package of claim 1, wherein the piezoelectric films comprise amaterial selected from the group consisting of Ba_(x)Sr_((1-x))TiO₃(BST), AlN and Al_(x)Ga_((1-x))N.
 3. The filter package of claim 1,wherein the piezoelectric films have a thickness of up to 2 microns. 4.The filter package of claim 1, wherein the piezoelectric films eachcomprise a single crystal.
 5. The filter package of claim 1 wherein thelower electrodes comprise aluminum.
 6. The filter package of claim 5wherein edges of the lower electrodes are stiffened by an under bumpmetallization material comprising a titanium adhesion layer followed byat least one of tungsten and tantalum and molybdenum.
 7. The filterpackage of claim 6 wherein an under bump metallization materialcomprising a titanium adhesion layer and a tungsten, tantalum ormolybdenum layer connects the lower electrode to the copper pillars. 8.The filter package of claim 1 wherein the lower electrode is coupled tothe interposer by solder tipped copper pillars.
 9. The filter package ofclaim 1 wherein the upper electrodes comprise at least one metal layerselected from the group comprising aluminum, titanium, tungsten, goldand gold-indium layers and a silicon layer.
 10. The filter package ofclaim 1 wherein the upper electrodes are multilayer electrodescomprising at least one metal layer with a relatively low DC resistanceand a second metal layer with a relatively high acoustic impedance. 11.The filter package of claim 10 wherein the at least one metal layer witha relatively low DC resistance comprises aluminum and is proximal to thepiezoelectric layer, and the second metal layer with a relatively highacoustic impedance comprises tungsten or molybdenum.
 12. The filterpackage of claim 11 wherein the upper electrode further comprises abonding layer for attachment of the relatively high acoustic impedancelayer with the silicon membrane.
 13. The filter package of claim 1wherein the upper electrode comprises either a tungsten or molybdenumlayer proximal to the piezoelectric film and a bonding layer between thetungsten or molybdenum layer and the silicon membrane.
 14. The filterpackage of claim 1, wherein the passivation material is selected fromthe group consisting of polyimide, Benzocyclobutene (BCB), SiO₂, Ta₂O₅,and Si₃N₄.
 15. The filter package of claim 1 wherein the upper cavitiesbetween the silicon membrane and the silicon handle have side wallscomprising residual silicon oxide.
 16. The filter package of claim 1wherein the cavities between the array of silicon membrane and thecommon silicon cover have a depth of between 3 and 10 microns.
 17. Thefilter package of claim 1 wherein the silicon membranes of the array ofupper electrodes are separated by a band of the passivation materialthat is 3 to 10 microns wide and there is a stepped interface within thesilicon layer.
 18. The filter package of claim 1 further comprising afirst adhesion layer between the piezoelectric membrane and the lowerelectrode.
 19. The filter package of claim 18 wherein the first adhesionlayer comprises titanium.
 20. The filter package of claim 1 wherein thesilicon wafer is attached to the upper electrode by an adhesion layeradjacent to the upper electrode, a bonding layer and a further adhesionlayer attached to the silicon wafer thereby creating a compositeelectrode.
 21. The filter package of claim 20 wherein the adhesion layerand further adhesion layer comprise Ti having a thickness in the rangeof 5 nm to 50 nm with 5% tolerances.
 22. The filter package of claim 20wherein the bonding layer is selected from the group comprising Au—Inalloy, Au and AlN.
 23. The filter package of claim 1 wherein the siliconmembrane is single crystal silicon with orientation of <111> or <100> or<110>.
 24. The filter package of claim 1 wherein the silicon membranehas a thickness in the range of 1 micron to 10 microns and is coupled tothe silicon cover by silicon oxide.
 25. The filter package of claim 24wherein the silicon cover is below 150 microns thick.
 26. The filterpackage of claim 24 wherein the silicon cover over the cavity isperforated with through silicon via holes.
 27. The filter package ofclaim 1 encapsulated in polymer over-mold/under-fill (MUF), wherein abarrier between the filter array and the interposer provides a perimeterwall of the lower cavity, wherein said barrier comprises at least one ofan SU8 gasket around the filter array that is attached to the lowerelectrode and an epoxy dam attached to the interposer.
 28. The filterpackage of claim 1, wherein the interposer comprises at least one vialayer and one routing layer of copper encapsulated a dielectric matrix.29. The filter package of claim 28, wherein the interposer comprises apolymer matrix selected from the group consisting of polyimide, epoxy,BT (Bismaleimide/Triazine), Polyphenylene Ether (PPE), PolyphenyleneOxide (PPO) and their blends.
 30. The filter package of claim 28,wherein the interposer comprises a Low Temperature Cofired Ceramic(LTCC) matrix.
 31. The filter package of claim 28, wherein theinterposer further comprises glass fibers and/or ceramic fillers.